1. Field of Invention
Aspects of this invention are related to sensing information to characterize a minimally invasive surgical instrument, and more particularly to shape sensors used to obtain shape information and absolute three-dimensional pose information for the minimally invasive surgical instrument.
2. Related Art
A recurring problem in minimally invasive surgery is determining the pose and/or shape of an instrument used in the surgery. Also, determining anatomical locations in a patient and correlating MRI images, or other images to images of the patient taken during surgery requires three-dimensional measurements.
Current methods used to obtain shape information about a flexible surgical instrument include camera and vision based methods, inertial and gyroscopic sensors based methods, and electromagnetic field sensor based methods. Also, there are mechanical systems that can touch two different locations and determine the relative relationships between the two locations.
These methods are limited in sensing precision and accuracy, sensitivity to environmental conditions, invasiveness in practical application, and can generally only provide position information on a certain discrete number of points on the surgical instrument. Also, the distances measured are relative distances and not absolute distances.
Optical fiber shape sensors are known. See e.g., U.S. Pat. No. 5,798,521 (filed 27 Feb. 1997), U.S. Pat. No. 6,389,187 B1 (filed 17 Jun. 1998), U.S. Patent Application Pub. No. US 2006/0013523 A1 (filed 13 Jul. 2005), and Roger G. Duncan et al., Characterization of a Fiber-Optic Shape and Position Sensor, Smart Structures and Materials 2006: Smart Sensor Monitoring Systems and Applications (D. Inaudi et al. eds.), 6167 Proceedings of SPIE 616704 (16 Mar. 2006), all of which are incorporated by reference. Optical fiber shape sensors have been used to sense joint angles in a minimally invasive surgical instrument. See, e.g., U.S. Patent Application Pub. No. US 2007/0156019 A1 (filed 20 Jul. 2006) and NASA Tech Briefs, NASA-Inspired Shape-Sensing Fibers Enable Minimally Invasive Surgery (1 Feb. 2008), http://www.techbriefs.com/content/view/2585/, both of which are incorporated herein by reference.
Typically, optical fiber shape sensors operate via optical time domain reflectometry (OTDR) or via optical frequency domain reflectometry (OFDR). Most approaches use a backscatter method that measures changes in backscattered light caused by a change in the shape of the optical fiber. Scattering mechanisms that have been used include Rayleigh scattering, Raman scattering, Brillouin scattering, and Fluorescence scattering. Fiber Bragg Gratings and the Kerr effect have also been used in shape-sensing sensors. Some of the Fiber Bragg Grating shape sensors are insensitive to twist. Consequently, if the shape sensor is twisted, the pose information obtained using the twisted shape sensor is not correct.
Other techniques have also been used for shape sensing. For example, electromagnetic fields have been used in combination with sensors mounted on an endoscope to determine a position of an endoscope. See U.S. Patent Application Publication No. 2007/0249901 A1 (filed Mar. 28, 2006; disclosing “Instrument Having Radio Frequency Identification Systems and Methods for Use.”), which is incorporated herein by reference in its entirety. Also, the changes in resistance of a piezoresistive flexible shape sensor have been used to measure shape changes.